This invention relates to methods and apparatus for testing erbium optical amplifiers.
Recent developments in erbium amplifier technology, particularly driven by the expansion of the wave division multiplexed (WDM) bandwidth over which gain is required, have highlighted issues with historic amplifier measurement methods. As amplifier designs have moved from xe2x80x9cred bandxe2x80x9d (xcx9c1540-1560 nm) to xe2x80x9cfull bandxe2x80x9d (xcx9c1530-1565 nm) and eventually xe2x80x9cextended bandxe2x80x9d (xcx9c1570-1606 nm) non-linear physical phenomena have become increasingly important. Perhaps the most important of these in this context is spectral hole burning. In this phenomenon, the presence of a strong input signal xe2x80x9cburnsxe2x80x9d a hole in the gain spectrum near to the wavelength of the input signal. This not only distorts the gain spectral shape, but also alters the operating condition, thereby inducing dynamic gain tilt into the spectrum. Such spectral hole burning is only significant if much of the input signal is concentrated in a small portion of the spectrum. This occurs, for example, in measurements using the xe2x80x98noise gain profilexe2x80x99 technique, in which the amplifier is placed in its saturated condition using a single saturating wavelength. This measurement techniques is therefore particularly prone to errors caused by spectral hole burning.
FIG. 1 shows a comparison of measurements made using the prior art noise gain profile technique and demonstrates spectral hole burning with an ideal spectrum. In this case, non-ideal extinction of the saturating signal leads to noisy peaks at the saturating signal""s wavelengths (1530 nm and 1550 nm respectively).
It will be appreciated that for accurate measurements, the test source used must accurately represent the multi-channel loading which the amplifier will experience within the real world system for which the amplifier is designed.
The channel wavelengths and spacings used in a WDM network have been standardised by the International Telecommunications Union (ITU) which has defined a so-called xe2x80x9cITU Gridxe2x80x9d which presently specifies channels spaced 100 GHz apart typically in the 1611.79 nm to 1492.2 nm wavelength range (186.00 THz-200.90 THz) spreading from a reference wavelength of 1552.52 nm (193.1 THz).
One approach to the measurement of an erbium amplifier for such use is comparative measurement using a grid of signals selected from the ITU grid with varying channel separations. Populating the grid completely with wavelength stabilised channels separated by 100 GHz or less is expensive. Using a sub-populated grid can give a reasonable accuracy but reduces the spectral resolution available. Thus in the prior art, there is a trade-off between accuracy/resolution and cost.
The spectral loading used to test erbium amplifiers needs to closely resemble conditions seen in the field. Use of an ITU grid is a good way of achieving this, however it is a costly solution and may restrict the spectral resolution. Use of a broadband source such as a high power amplified spontaneous emission source gives a cost-effective alternative, which compares well with ITU grid measurements. In addition the spectral resolution of the measurement is then only limited by the spectral resolution of the optical spectrum analyser.
Thus, in a first aspect, the invention provides a method of testing an optical amplifier comprising the steps of coupling an ASE source to the amplifier input, and measuring the amplifier performance using an optical spectrum analyzer.
In a second aspect, the invention provides apparatus for testing an optical amplifier comprising a first terminal couplable to the input terminal of an amplifier under test, an ASE source having its output coupled to the first terminal, a second terminal couplable to the output of an amplifier under test, and a third terminal coupled to the second terminal and couplable to an optical spectrum analyzer.
The term xe2x80x98optical spectrum analyzerxe2x80x99 as used herein encompasses a measuring instrument operable to analyze optical characteristics in a relatively narrow frequency band which does not necessarily cover the whole of the operating spectrum of the device under test.